An energy harvesting circuit can be used to harvest energy from an energy source and charge a battery using the harvested energy. There are conventional energy harvesting circuits configured to harvest energy at very low power levels if the energy source voltage is the same or higher than the battery voltage. There are also known energy harvesting configurations for harvesting energy at levels down to about 50 uW when the harvesting source voltage is 8 to 20 times lower than the output (battery) voltage. Current needs for energy harvesting are at or below a few microwatts, for example about 2 uW, and trending downward. Smaller sized energy sources generate low power levels at low voltage levels. An example of such an energy source is a small solar cell that generates about 2 uW and about 0.5V. A problem lies in how to harvest energy at a voltage level that is much less than the battery voltage, and to boost the harvested voltage to the battery voltage in an efficient means at those power levels.
FIG. 1A illustrates a conceptual schematic diagram of a conventional energy harvesting circuit configured to harvest energy from a low power source at low voltage to charge a battery. The energy harvesting circuit 10 includes a low voltage source 20, a storage capacitor C1, an inductor L1, a transistor T1, a diode D1, a capacitor C2, a boost controller circuit 30, a battery 40, a comparator 50, a resistor R1 and a resistor R2. The boost controller circuit 30, the transistor T1, the inductor L1, the diode DL and the capacitor C2 form a boost converter circuit. The low voltage source 20 is a low level power source, conceptually represented as voltage source 22. In an exemplary application, the low voltage source 20 generates 2 uW at 0.5V. The low voltage source 20 also has a source impedance, conceptually represented as resistor RS. As used herein, reference to a “source voltage” refers to voltage VS across the voltage source 22. The transistor T1 functions as a switch that enables current flow from the storage capacitor C1 to the inductor L1 and then to the battery 40, this configuration is typically called a boost converter. When the transistor T1 is turned on, the voltage at VC is applied across the inductor L1 allowing energy to be stored in the inductor L1. While the transistor T1 is on, the diode D1 is reverse biased, thereby blocking the battery 40 voltage and not allowing current to flow out of the battery 40. When the transistor T1 is turned off, the stored energy in the inductor L1 flows through the diode D1 and delivers energy into the battery 40. The capacitor C2 is in parallel with the battery 40 and is used to reduce the impedance of the battery 40 at the switching frequency, and thus filter out pulses of energy coming from the diode D1.
The boost controller circuit 30 supplies a control signal as a gate voltage to the transistor T1, thereby turning the transistor T1 on and off. The boost controller circuit 30 provides a Pulse Width Modulated (PWM) signal to the gate of the transistor T1, thus modulating the amount of energy that is delivered to the battery 40. In the exemplary energy harvesting circuit of FIG. 1A using this type of boost controller, the duty cycle is fixed and the output of the boost controller is regulated by means of a Pulsed Frequency Modulation (PFM) input. The PFM signal regulates the output within a voltage window that is set by an amount of hysteresis in comparator 50 and the reference (REF) input. In other examples, the duty cycle of the PWM output is controlled by a circuit inside of the boost controller circuit 30 that changes the duty cycle as a function of the output voltage when compared to an internal reference. In the example of FIG. 1A, the boost controller circuit 30 is turned on and off by the PFM Enable signal supplied by the comparator 50. A first input of the comparator 50 is coupled to an output of the battery 40. A second input of the comparator 50 is coupled to a reference voltage. The output of the comparator 50 goes low when the battery voltage is above the reference voltage and this is set to be at the fully charged battery voltage. The output of the comparator 50 does not go high until the battery voltage is reduced to a minimum voltage level that is less than the reference voltage due to hysteresis built into the comparator 50. For battery charging, if the battery 40 is at the regulation voltage, then there would be no need for harvesting since the battery would already be charged.
The energy harvesting circuit 10 uses pulse frequency modulation (PFM) to harvest the low power level generated by the low voltage source 20. This is accomplished by monitoring the boosted output voltage across the battery 40 with the comparator 50. However, this type of converter achieves low power operation only when the output is at its desired regulation level and this is when the battery is fully charged. In order to charge the battery 40, the boost converter of FIG. 1A consumes too much power and can not achieve harvesting at 2 uW level. Whenever the battery 40 is lower that the preset voltage level (fully charged) determined by the resistors R1, R2 and the reference voltage (REF), the output of the comparator 50 remains high allowing the boost controller 30 to run continuously. For a battery that is below the regulation level and needs to be charged, the boost controller 30 of FIG. 1A attempts to deliver more energy than there is available at the low voltage source 20, thereby dragging down the voltage at VC. Since the boost controller 30 requires supply current that comes from the battery 40 to perform its function, and if the voltage at VC is too low to allow sufficient energy to be replaced, the boost converter would remove more energy from the battery than it would deliver into the battery.
A disadvantage of the energy harvesting circuit 10 is that in order to harvest energy of a few microwatts at low voltage, it is necessary to consume less energy out of the battery to operate the boost controller 30 compared to the amount of energy being delivered back into the battery. Even assuming that the supply current problem can be overcome, there are other problems with the energy harvesting circuit of FIG. 1A. In order to harvest low power at the 2 uW level, the boost controller 30 needs to have the average inductor L1 current be equal to the average current available from the low voltage source 20. One way to achieve this low current is to make the inductor L1 very large, which is undesirable for most applications. Another way to make the average inductor current very low is to make the converter operate at a very low duty cycle, but this limits the boost converter to one power level and increases the supply current. In an exemplary application, the low voltage source 20 generates 2 uW at 0.5V, and the battery 40 has a charged voltage of 4V. To harvest 2 uW at 0.5V the supply current from the 4V battery to the boost controller 30 needs to be about 100 nA, the drain to source capacitance of the transistor T1 needs to be about 0.1 pF, and the impedance of the inductor L1 needs to be about 40 mH. Typical supply currents for boost converters are in the 10 uA to 100 uA range so it is unrealistic to harvest energy below 40 uW.
The comparator 50 monitors the output of the battery 40, and if the battery voltage is greater than a reference voltage, the PFM Enable signal is low and the boost converter circuit is off. Even with the boost converter circuit off, the comparator 50 still requires power to operate and perform the comparison function. Although the energy harvesting circuit 10 provides a pulse modulation means for periodically turning on and off the boost converter circuit, this architecture is ineffective for charging the battery 40. Using the energy harvesting circuit 10, if the battery 40 needs to be charged, for example the monitored battery output is less than the reference voltage, then the boost controller circuit 30 will always be turned on and the boost converter circuit will always be consuming more power than it can deliver to the battery 40. The low voltage source 20 does not generate enough power to continuously power the boost converter circuit. The boost converter circuit would only turn on and drag down the voltage VC while harvesting, and then consume more power from the battery 40 than is able to be delivered from the low voltage source 20.
The PFM type method utilized in FIG. 1A is useful for turning the boost converter circuit on and off, but the energy harvesting circuit is not applicable to charging a battery. Instead, the PFM type method used in the energy harvesting circuit 10 is more useful for consuming energy from a battery and supplying energy pulses to a load. For example, the energy harvesting circuit of FIG. 1A is adapted to replace the low voltage source 20 with a battery, and replace the battery 40 with a load that requires power delivery of only a couple microwatts. In this configuration, the boost converter circuit can deliver only a couple microwatts, and shut down when not needed. Although useful for providing energy bursts to a load, the energy harvesting circuit described in FIG. 1A does not provide an effective means for charging a battery.
FIG. 1B illustrates a conceptual schematic diagram of a conventional energy harvesting circuit configured to harvest energy from a power source to charge a battery. The energy harvesting circuit 80 includes a power source 24, a storage capacitor C5, an inductor L3, a diode D3, a capacitor C7, a converter controller circuit 60, a battery 42, a capacitor C6, resistors R3-R10, and an enabling circuit 70. The converter controller circuit 60, the inductor L3, the diode D3, the capacitor C6, and the capacitor C7 form a converter circuit. Energy from the solar cell power source 24 is stored in the capacitor C5. The converter controller circuit 60 includes similar functionality as the boost controller circuit 30 and the transistor T1 of FIG. 1A to enable or disable current flow from the capacitor C5 to the inductor L3, and from the inductor L3 to the battery 42. The converter controller circuit 60 is enabled and disabled by the enabling circuit 70, which includes a comparator 72, a comparator 74, and a diode D4. A reference voltage is input to each of the comparators 72 and 74. The voltage of the capacitor C5 is monitored by the comparator 72 and compared to a reference voltage. Once the capacitor voltage reaches the reference voltage, the comparator 72 outputs an enabling signal to the converter controller circuit 60. This initiates discharging of the capacitor C5 for charging of the battery 42. Once the capacitor C5 is discharged to a predetermined minimum voltage level, the comparator 72 outputs a disable signal to the converter controller circuit 60, which in turn is turned off thereby stopping discharge of the capacitor C5.
The comparator 74 monitors the battery voltage and disables the converter controller circuit 60 when the battery voltage reaches a high limit and enables the converter controller circuit 60 when the battery voltage declines below a predetermined level. The dual comparator with hysteresis controls shutdown of the converter controller circuit 60 and also the selection of two different charging rates, such as a fast charge and a trickle charge. During fast charge, the converter controller circuit 60 operates as a current source, forwarding inductor energy from the inductor L3 to the battery 42 without checking the battery voltage. When charging a discharged battery 42, the energy harvesting circuit 80 applies full fast-charge current until the battery voltage reaches its upper limit. The converter controller circuit 60 is then disabled until the battery voltage declines to the next-lowest limit, whereupon the converter controller circuit 60 is enabled for the trickle charge. Trickle charging continues until the battery voltage reaches its upper limit, whereupon the converter controller circuit 60 is disabled, or its lower limit, whereupon the converter controller circuit 60 is enabled for fast-charge again.
As shown in FIG. 1B, the enabling circuit 70 is provided supply voltage V+ from the battery 42. Further, the resistor divider R9, R10 is coupled to the battery 42. As such, the enabling circuit 70 and the resistor divider R9, R10 act as loads on the battery when the converter controller circuit 60 is turned off, or when the energy harvesting circuit 80 is not connected to the solar cell power source 24. Although the energy harvesting circuit 80 may be configured for minimal battery discharge during these conditions, the energy harvesting circuit 80 is ineffective for those applications requiring the energy harvesting circuit 80 to apply zero load to the battery when disabled.